Medicare Spending for Previously Uninsured Adults

  1. J. Michael McWilliams, MD, PhD;
  2. Ellen Meara, PhD;
  3. Alan M. Zaslavsky, PhD; and
  4. John Z. Ayanian, MD, MPP
  1. From Harvard Medical School, Brigham and Women's Hospital, and Harvard School of Public Health, Boston, Massachusetts, and National Bureau of Economic Research, Cambridge, Massachusetts.
     
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    Abstract

    Background: Medicare spending after age 65 years may be higher for previously uninsured adults if suboptimal care before this age leads to irreversible complications, persistently elevated clinical risks, or delay of costly elective procedures.

    Objective: To compare Medicare spending for previously uninsured and insured adults by using Medicare claims data.

    Design: We used longitudinal survey data and linked Medicare claims data to compare Medicare spending for beneficiaries age 65 to 74 years who were previously insured or previously uninsured before age 65 years. We used an inverse-probability-of-treatment weighting technique to adjust for fixed and time-varying sociodemographic and health characteristics before age 65 years. We also compared condition-specific hospitalizations and estimated their contribution to differences in Medicare spending.

    Setting: Nationally representative Health and Retirement Study, 1992–2006.

    Participants: 2951 continuously insured adults and 1616 adults who were continuously or intermittently uninsured before age 65 years.

    Measurements: Mean adjusted annual Medicare spending (total and by type of service) and annual rates of condition-specific hospitalizations.

    Results: Adjusted annual total Medicare spending was significantly higher for previously uninsured ($5796) than previously insured ($4773) adults (difference, $1023 [95% CI, $29 to $2016]; P = 0.044). Among relevant clinical subgroups, previously uninsured adults had higher adjusted annual hospitalization rates than previously insured adults for complications related to cardiovascular disease or diabetes (9.1% vs. 6.4%; P = 0.002) and for joint replacements (2.5% vs. 1.3%; P = 0.006). Differences in these hospitalizations accounted for 65.7% of the $644 difference in annual Medicare inpatient spending between all previously uninsured and insured adults.

    Limitation: Unobserved confounders could have explained spending differences.

    Conclusion: Costs of expanded coverage before age 65 years may be partially offset by subsequent reductions in Medicare spending after age 65 years, particularly for uninsured adults with cardiovascular disease, diabetes, or severe arthritis.

    Primary Funding Source: The Commonwealth Fund.

    Editors' Notes

    Context

    • Most proposals for health care reform in the United States provide insurance coverage for uninsured people.

    Contribution

    • Medicare spends more for people who were uninsured before they become eligible for Medicare than for people who were insured before they become eligible for Medicare ($5796 vs. $4773 per year during the 10 years after joining Medicare).

    Caution

    • Unobserved confounders might explain the difference.

    Implication

    • Insurance coverage for uninsured adults might be a more valuable investment for the United States than previously thought.

    –The Editors

    The burden of chronic disease has increased in the United States in the past decade (1), and many adults with chronic conditions lack health insurance (2, 3). As rising health care costs and a slowing economy threaten to increase the number of uninsured adults (4, 5) and thwart efforts to expand coverage (6), assessments of the value of health insurance for chronically ill adults may help to guide health care reform.

    Growing evidence suggests that health insurance coverage benefits the health of adults with treatable conditions (7, 8). For uninsured adults with cardiovascular disease or diabetes in particular, near-universal Medicare coverage after age 65 years increases use of health services (9–12), improves self-reported health trends (13), and reduces disparities in clinical measures of disease control (14). Therefore, previously uninsured Medicare beneficiaries may have greater morbidity and require costlier care than if they had been continuously insured before age 65.

    In a previous national study of adults age 21 to 64 years, medical expenditures for uninsured adults who acquired insurance coverage were not subsequently higher than expenditures for continuously insured adults (15). However, uninsured adults in this study were predominantly young and healthy, only a small fraction of them gained coverage, and changes in insurance status may have been driven by individuals' choices and financial means. In contrast, nearly all Americans become eligible for Medicare at age 65 years. Moreover, because of a much higher prevalence of chronic disease, differences in Medicare spending between previously uninsured and insured adults may be much larger, but these differences have not been quantified by using Medicare claims. Consequently, these potential economic consequences to the Medicare program have not been considered in estimating the costs of expanding coverage for uninsured middle-aged adults (16, 17).

    Medicare spending may be higher for previously uninsured adults with cardiovascular disease and diabetes if deficient care leads to irreversible complications before age 65 years (18–22) or to elevated clinical risks that persist after age 65 years (23, 24). Uninsured adults may also delay costly elective procedures, such as joint replacements for severe arthritis, until they are eligible for Medicare. If differences in Medicare spending for previously uninsured and insured adults are driven by clinical conditions and services that reflect these hypothesized effects, then differences in spending after age 65 may be attributed to differences in insurance coverage before age 65 with greater confidence.

    Using longitudinal national survey data linked with Medicare claims, we compared Medicare spending after age 65 for previously uninsured and insured adults. We expected hospitalizations for complications related to cardiovascular disease or diabetes and for joint replacements to be particularly influenced by previous insurance coverage. Therefore, we also compared rates of these condition-specific hospitalizations and estimated their contributions to differences in Medicare spending for previously uninsured and insured adults. On the basis of these analyses, we estimated reductions in Medicare spending from age 65 to 74 that could be achieved if all Americans currently 51 to 64 years old were continuously insured before age 65.

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    Methods

    Study Population

    We analyzed data from the Health and Retirement Study, a nationally representative, longitudinal survey (25). In 1992, this study enrolled noninstitutionalized adults and eligible spouses age 51 to 61 in the continental United States. Initial interviews conducted in 7705 households yielded 9749 participants (82% response rate). Biennial surveys were conducted through 2006, when participants were age 64 to 74. In each survey, participants who were eligible for Medicare were asked to provide their Medicare identification numbers for linkage to Medicare claims files available through 2005. Our study cohort included participants who were age 65 or older by 2005, were linked to their enrollment files, and were not continuously enrolled in Medicare managed care after age 65. We excluded adults who reported public insurance coverage in 1992 because insurance coverage before 1992 was not observed, and preceding episodes of lack of insurance for some of these adults may have led to medical conditions, such as end-stage renal disease, that qualified them for public insurance and were not measured in the survey. Without this exclusion, higher subsequent health care spending for qualifying conditions present in 1992 would be inaccurately associated with a group uniformly classified as insured. In contrast, preceding insurance coverage was known for all subsequent transitions to public insurance, which were therefore included in the analysis.

    Our study protocol was approved by the Harvard Medical School Committee on Human Studies and by the Privacy Board of the Centers for Medicare & Medicaid Services.

    Study Variables

    We used biennial survey data from 1992 to 2006 to assess longitudinal patterns of insurance coverage and sociodemographic and health characteristics before age 65. In each survey, participants answered detailed questions about sources of health insurance and, if insured, whether they lacked coverage at any time in the previous 2 years. As in earlier research (13), we classified participants as previously insured if they reported a continuous pattern of initial private coverage and subsequent private or public coverage in all surveys before age 65, and as previously uninsured if they reported continuous or intermittent uninsurance before age 65. After age 65, participants were classified as having supplemental insurance if they ever reported having employer-sponsored insurance, Medicaid, Medigap, or traditional Medicare with additional coverage. We determined vital status after age 65 and enrollment in Medicare Part B and Medicare managed care from Medicare denominator files.

    For each participant, we used claims data from 1996 to 2005 to assess annual total Medicare spending after age 65 and spending in 7 distinct categories of service. We determined spending from reimbursements by Medicare to providers, inflated to constant 2006 U.S. dollars using the gross domestic product deflator (26, 27). For years in which adults were enrolled in Medicare managed care for less than 12 months, we inflated partial-year spending in traditional Medicare to annual estimates. We also determined annual utilization from claims data for 4 general services: hospital stays, outpatient institutional visits, physician office visits, and days in skilled-nursing facilities.

    We assessed annual indicators of hospitalization for 2 prespecified categories of diagnosis-related groups (DRGs): 1) complications related to cardiovascular disease or diabetes (DRGs 14 to 17, 36, 87, 103, 106, 107, 109 to 111, 113 to 118, 121 to 125, 127, 129 to 134, 138 to 140, 143 to 145, 294, 316 to 317, 478 to 479, 515 to 518, 524 to 527, 533 to 536) and 2) lower-extremity joint replacement (DRGs 209 and 471). These condition-specific hospitalizations were assessed for the entire study cohort and for 2 high-risk clinical subgroups: 1) adults reporting cardiovascular disease (hypertension, heart disease, stroke) or diabetes by age 67 and 2) adults reporting arthritis by age 67. We used this age threshold to allow adults with undiagnosed conditions to be diagnosed and report conditions in a biennial survey within 3 years of age eligibility for Medicare.

    Statistical Analysis

    Because intermittent uninsurance is common among near-elderly adults and predicts adverse health outcomes (13, 28–30), we defined cohorts by longitudinal assessment of insurance coverage over multiple surveys before age 65 rather than by a single cross-sectional assessment. Our cohort definitions (continuously insured vs. continuously or intermittently uninsured) thus provided greater statistical power for comparisons of subsequent Medicare spending but also meant that we had to control for time-varying confounders. For example, uninsurance before age 65 may have caused health declines, in turn leading to higher Medicare spending, but declining health also may have caused uninsurance. If we did not control for health declines before the onset of uninsurance, we might mistakenly attribute higher Medicare spending to the effects of uninsurance before age 65 when higher spending was actually the result of earlier deteriorations in health.

    A previous longitudinal analysis of these data demonstrated that coverage losses predicted subsequent health declines, but health declines did not predict coverage losses (31). Thus, effects of uninsurance on health are likely to predominate over the effects of health on uninsurance in this age group. Nevertheless, we used an inverse-probability-of-treatment weighting technique to adjust comparisons of previously uninsured and previously insured adults for both fixed and time-varying characteristics (Appendix, available at www.annals.org) (32, 33). After the weighting adjustment, the likelihood of being uninsured at any given survey before age 65 was independent of the sociodemographic and health characteristics (listed in Table 1) observed up to that time. Because insured and uninsured adults had similar weighted distributions of health measures at the time of survey assessments before age 65, the weighting adjustment minimized differences in insurance status due to health declines before each survey while preserving differences in health that subsequently developed from differences in insurance status. Thus, we distinguished effects of uninsurance before age 65 on health from potentially confounding effects of health on uninsurance.

    View this table:
    Table 1. Characteristics of the Study Cohort by Insurance Coverage

    In our main analyses, we sought to compare previously uninsured and insured adults who had equivalent access to care after age 65. Therefore, we also included an indicator of supplemental coverage when we constructed inverse-probability-of-treatment weights. Using a similar weighting method, we adjusted for survey and item nonresponse that led to missing Medicare identification numbers and claims data (34).

    To estimate absolute differences between previously uninsured and insured adults, we fitted linear models predicting annual spending or utilization as a function of insurance coverage before age 65 and applied the weights we had constructed. For these analyses, we stratified the study cohort into adults with and without cardiovascular disease or diabetes by age 67 on the basis of self-reports. We also fitted logistic models for annual indicators of condition-specific hospitalization among the high-risk subgroups with cardiovascular disease or diabetes or arthritis. In our models of spending and utilization, we adjusted for geographic variation in spending by including fixed effects for metropolitan statistical areas or non–metropolitan statistical area counties because uninsured adults might have been disproportionately located in areas of higher or lower health care spending. We adjusted all analyses for the complex survey design (35) and repeated measures using sampling weights and robust design-based variance estimators (36).

    We assessed the contribution of condition-specific hospitalizations to differences in Medicare spending by comparing spending differences between previously uninsured and insured adults before and after adding annual counts of these hospitalizations as predictors to models. We also added the development of end-stage renal disease as an explanatory predictor.

    For previously uninsured and insured adults with cardiovascular disease or diabetes, we expected differences in health care needs to persist after age 65 but to diminish with age as disease control, physical functioning, and the risk for adverse cardiovascular outcomes improved for previously uninsured adults (13, 14). Therefore, in exploratory analyses we examined age profiles of Medicare spending and condition-specific hospitalizations to determine the ages at which differences narrowed, and we added to the models interactions between previous insurance coverage and these ages.

    We conducted several sensitivity analyses. To determine whether differential selection into managed care may have biased our results, we further adjusted analyses using weights derived from a predictive model of continuous enrollment in managed care after age 65. Because health care provided by the Veterans Health Administration is not captured in Medicare claims, we excluded all military veterans. We also excluded observations for beneficiaries when they were not enrolled in Medicare Part B. For rare instances in which Medicare was not the primary payer for Medicare-covered services (3.6% of beneficiary-years), we added reimbursements from other primary payers to spending measures and then repeated the main analyses. Finally, we also compared annual rates of hospitalization for chronic obstructive pulmonary disease, a condition for which we hypothesized that uninsurance before age 65 would not lead to persistently greater health care needs after age 65 (Appendix).

    All analyses were performed by using SAS statistical software, version 9.1 (SAS Institute, Cary, North Carolina). We report 2-sided P values without adjustment for multiple testing.

    Role of the Funding Source

    This study was funded by the Commonwealth Fund. The funding source had no role in the design, conduct, or reporting of the study or in the decision to submit the manuscript for publication.

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    Results

    Of the 9749 participants interviewed in 1992, we excluded 741 (7.6%) who died before age 65 and 971 (10.0%) who did not reach age 65 by 2005. Medicare identification numbers were not available for 1141 (11.7%) who did not respond to surveys after age 65. Of the remaining 6896 participants, 5377 (78.0%) provided their identification numbers and were linked to their enrollment files. Rates of missing Medicare claims data due to survey and item nonresponse were similar for previously uninsured and insured adults (32.6% vs. 32.2%; P = 0.74). Of participants with linked claims files, we excluded 450 (8.4%) who had public insurance coverage in 1992 and 360 (6.7%) who were continuously enrolled in Medicare managed care after age 65. Of the remaining 4567 participants who were age 52 to 61 in 1992 and age 65 to 74 in 2005, 2951 (64.6%) reported continuous coverage, and 1616 (35.4%) reported being uninsured before age 65 (for 44% of the surveyed years on average).

    Adults who were uninsured before age 65 differed significantly from continuously insured adults in many fixed and time-varying sociodemographic and health characteristics (Table 1). After age 65, previously uninsured adults were less likely to have supplemental insurance coverage than previously insured adults, but enrollment in Medicare managed care and Part B coverage did not differ between these groups (Table 1).

    Mean adjusted annual Medicare spending was significantly higher for previously uninsured adults than for previously insured adults (Table 2), with differences most pronounced in the highest quartiles of their spending distributions (for previously uninsured vs. previously insured adults, median, $707 vs. $706; 75th percentile, $3108 vs. $2608; 90th percentile, $12 463 vs. $9406; 95th percentile, $26 787 vs. $21 813, respectively). Differences in total Medicare spending were largely due to differences in inpatient and home health spending (Table 2) and were concentrated among the 66.7% of adults with cardiovascular disease or diabetes (Figure 1). These results were not substantially changed in sensitivity analyses (data not shown). Descriptive plots and post hoc testing suggested that differences in total Medicare spending diminished after age 71 (P = 0.104 for comparison of age 65 through 71 to age 72 through 74).

    View this table:
    Table 2. Adjusted Annual Medicare Spending and Utilization After Age 65 by Type of Service, Insurance Coverage Before Age 65, and History of Cardiovascular Disease or Diabetes
    Figure 1.
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    Figure 1. Total Medicare spending by age, insurance coverage before age 65, and history of cardiovascular disease (CVD) or diabetes.

    Mean adjusted total Medicare spending is plotted by age and insurance coverage before age 65 for adults with (top) and adults without (bottom) a reported diagnosis of hypertension, heart disease, stroke, or diabetes by age 67. Annual Medicare spending after age 65 was significantly higher for previously uninsured adults than previously insured adults with cardiovascular disease or diabetes, and this difference was particularly large for those age 65 to 71 (adjusted difference, $1400; P = 0.027) (top). Annual Medicare spending after age 65 did not differ for previously uninsured and insured adults without cardiovascular disease or diabetes (bottom). Plots were truncated from age 74 to age 72 because of diminished sample sizes for estimates after age 72. Standard error bars are provided for plotted means.

    Previously uninsured adults had more hospital stays and outpatient institutional visits annually than previously insured adults, but not more physician office visits or days in skilled-nursing facilities (Table 2). Among adults with cardiovascular disease or diabetes, previously uninsured adults were more likely to be hospitalized for complications related to cardiovascular disease or diabetes, and specifically for myocardial infarction, heart failure, or stroke (Table 3). Descriptive plots suggested that differences in hospitalization risk narrowed with age (Figure 2), but this narrowing was not statistically significant. Among adults with arthritis, previously uninsured adults were more likely to be hospitalized for joint replacement than were previously insured adults (Table 3).

    View this table:
    Table 3. Condition-Specific Hospitalizations After Age 65 for Previously Uninsured and Insured Adults
    Figure 2.
    View larger version:
    Figure 2. Annual rates of hospitalization for complications related to cardiovascular disease or diabetes by age and insurance coverage before age 65.

    Among adults with cardiovascular disease or diabetes, annual rates of hospitalization after age 65 for complications related to these conditions were significantly higher for previously uninsured adults than previously insured adults. This increased risk for previously uninsured adults diminished with increasing age, but not significantly (P = 0.38 for interaction between previous insurance coverage and age). Standard error bars are provided for mean rates.

    Of 5485 hospital admissions after age 65, 1764 (32.2%) were for complications related to cardiovascular disease or diabetes and 274 (5.0%) were for joint replacements. On the basis of annual counts of hospitalizations for complications related to cardiovascular disease or diabetes among previously uninsured and insured adults (0.16 per person vs. 0.10 per person; P = 0.04), 1 more of these hospitalizations occurred annually after age 65 for every 17 (1/0.06) previously uninsured adults with these conditions. Each of these hospitalizations was associated with an additional $11 000 in annual inpatient Medicare spending on average, as estimated when annual hospitalization counts were added as predictors to spending models. After adjustment for hospitalizations for complications related to cardiovascular disease or diabetes and for joint replacements, the adjusted difference in annual inpatient spending for previously uninsured and insured adults was reduced by 65.7% and was no longer statistically significant (from $644 per person to $221 per person; P = 0.29). Similarly, adjustment for these condition-specific hospitalizations and the development of end-stage renal disease reduced the adjusted difference in annual total Medicare spending by 62.9% (from $1023 per person to $380 per person; P = 0.25).

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    Discussion

    In this nationally representative cohort, adjusted Medicare spending was significantly higher for previously uninsured adults than previously insured adults. Consistent with clinical evidence (18–24), differences in inpatient spending were largest for adults with cardiovascular disease or diabetes because previously uninsured adults were substantially more likely than previously insured adults to be hospitalized after age 65 for costly complications related to these treatable conditions. Adjusted rates of hospitalization for joint replacement were also significantly higher for previously uninsured adults with arthritis, suggesting that uninsured adults may delay costly procedures that enhance physical functioning and quality of life (37–42). Despite accounting for just 37% of admissions, hospitalizations for complications related to cardiovascular disease or diabetes and joint replacements accounted for nearly two thirds of the adjusted difference in inpatient spending for all previously uninsured and insured adults.

    Higher Medicare spending after age 65 for previously uninsured adults suggests that the costs of expanding coverage for these adults may be lower than expected. For example, a recent analysis estimated that universal coverage would increase total annual medical spending for adults age 19 to 64 by $113 billion (16). However, this estimate did not account for potential reductions in subsequent spending after age 65 due to improved health from prevention of complications or to earlier use of elective procedures, such as joint replacements.

    For adults who are currently age 51 to 64, providing coverage to the uninsured for an average of 4 years (based on our study cohort) to achieve universal continuous coverage through age 65 would increase health care spending before age 65 by an estimated $197 billion (Appendix). Projecting from our findings, if the current population of adults age 51 to 64 had continuous insurance coverage from now through age 65, the present value of subsequent reductions in Medicare spending between age 65 and 74 for these adults would be approximately $98 billion (Appendix). Thus, from a societal perspective, subsequent reductions in Medicare spending after age 65 may offset almost half of this increased spending before age 65. Because Medicare pays for only 49% of all health care spending for elderly adults (43), the reduction in total medical spending after age 65 may be even greater.

    Our calculations assumed that expanding coverage would not affect mortality for uninsured near-elderly adults, but gains in life expectancy may be substantial for these adults (44–46). We compared increases in health care spending associated with the provision of insurance coverage before age 65 to subsequent decreases in spending after age 65 associated with better health. However, decreased annual spending due to better health may also be matched by increased lifetime spending due to greater longevity (47). Thus, survival gains for uninsured adults would add years of spending before and after age 65 to our calculated costs of expanded coverage before age 65 and thereby reduce the relative size of partially offsetting decreases in Medicare spending after age 65, particularly if these survival gains disproportionately increased the number of severely ill adults living beyond age 65. However, if providing insurance coverage to uninsured adults added valuable years to the lives of many near-elderly and elderly Americans, this would be an important benefit for policymakers to consider.

    Strengths of our study included detailed longitudinal survey data linked to Medicare claims, rigorous methods to adjust differences in Medicare spending for both fixed and time-varying confounders before age 65, and assessments of clinically relevant hospitalizations as contributors to these spending differences. Our findings are consistent with a previous analysis of self-reported hospitalizations after age 65 (10) and quasi-experimental studies suggesting distinctive health benefits of insurance coverage for near-elderly adults, particularly those with cardiovascular disease or diabetes (13, 14, 46, 48, 49). For adults with these chronic conditions, improvements in blood pressure, blood glucose, and cholesterol control associated with gaining coverage (14) may substantially reduce subsequent annual health care costs.

    Our study was subject to several limitations. Exploratory analyses suggested that differences in Medicare spending and complications related to cardiovascular disease or diabetes eventually narrowed with age, but we had limited statistical power for discriminating short-term from long-term effects of uninsurance on subsequent utilization and spending. Age-related narrowing evident in descriptive plots of hospitalizations for complications of cardiovascular disease or diabetes was consistent with relative health improvements for previously uninsured adults after acquisition of Medicare coverage (13). This narrowing was also consistent with previously demonstrated legacy effects of poorly controlled blood pressure and blood glucose among adults with diabetes, some of which persist indefinitely after better disease control is achieved (for example, higher risk for myocardial infarction due to previous lack of metformin therapy) and some of which diminish gradually over several years (for example, higher risk for stroke or microvascular complications due to poor previous blood pressure control) (23, 24). However, greater mortality rates among previously uninsured adult may have also contributed to narrowing differences (44). Because of limited sample sizes, we also could not precisely assess the contributions of other key conditions, such as cancer, to differences in Medicare spending between previously uninsured and insured adults.

    We could not observe health care utilization or spending for Medicare beneficiaries enrolled in managed care plans. However, managed care enrollment did not differ between previously uninsured and insured adults, and results were similar in analyses weighted to reflect the likelihood of continuous enrollment in Medicare managed care based on characteristics before age 65. Findings were similarly robust to sensitivity analyses addressing enrollment in Part B, veterans' benefits, and Medicare-covered services reimbursed by other primary payers.

    Finally, preexisting differences in unobserved characteristics between insured and uninsured near-elderly adults, such as health care preferences or disease severity, may have explained subsequent differences in adjusted Medicare spending. However, uninsured adults are likely to have weaker preferences for health care (50, 51), and their lower demand for care might persist after age 65. Therefore, differences in Medicare spending for previously uninsured and insured adults with equal preferences for care may be larger than our results suggest. Furthermore, if higher Medicare spending for previously uninsured adults with cardiovascular disease or diabetes was driven primarily by unmeasured differences in disease severity that were not caused by uninsurance before age 65, their elevated health care needs should have persisted indefinitely after acquisition of Medicare coverage. Yet, their risk for hospitalization for complications related to these conditions appeared to diminish relative to previously insured adults after several years of eligibility, coinciding with improved trends in health status demonstrated in a previous study (13). Finally, our inverse-probability-of-treatment weighting may have actually underestimated effects of uninsurance on subsequent Medicare spending if measured differences in health at the time of surveys were due to preceding differences in coverage.

    Our findings have important policy implications. Medicare spending between age 65 and 74 was significantly higher for previously uninsured adults than for previously insured adults who were otherwise similar in numerous measured characteristics before age 65. Greater spending for previously uninsured adults was largely explained by hospitalizations for complications related to cardiovascular disease or diabetes and for joint replacements. Thus, extending insurance to uninsured adults may result in substantial health gains for many older working-age adults, and subsequent reductions in Medicare spending after age 65 may partially offset increased spending from expanded coverage before age 65. These benefits suggest that health insurance coverage for uninsured adults over age 50 would be a more valuable investment for the United States than previously thought.

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    AppendixGraphic

     
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    Inverse-Probability-of-Treatment Weighting to Control for Time-Varying Confounders

    We used logistic regression to model insurance status at each biennial survey before age 65 as a function of numerous characteristics (listed in Table 1) assessed in the same survey and insurance status from the preceding survey. The cumulative products of predicted probabilities from this model yielded participants' probabilities of having their observed longitudinal coverage pattern given these fixed and time-varying characteristics. Individual weights were constructed from the inverse of these cumulative probabilities, and their variance was reduced by estimating stabilized weights as described elsewhere (32, 33). In recommended diagnostic checks (52), we examined the distribution of these stabilized weights to confirm that there were no extreme values and that the mean was close to 1.0 (mean, 1.0098). In a sensitivity analysis, we also truncated the distribution of these weights to the 5th and 95th percentiles. Estimated differences in Medicare spending remained statistically significant after this truncation. Finally, to check the robustness of weighted estimates to alternative modeling approaches (53), we added to spending models all baseline sociodemographic and health characteristics from 1992 that were already included in the predictive model used to derive inverse-probability-of-treatment weights.

    Assuming no unmeasured confounders, these weights approximated sequential randomization of insurance status at each survey before age 65, conditional on concurrent characteristics. Thus, whereas standard regression or propensity-score methods would allow comparisons of Medicare spending after age 65 by insurance status at a specific age or time before age 65 (for example, insured vs. uninsured at baseline in 1992) with adjustment for observed characteristics at that time, this weighting approach facilitated adjusted comparisons of Medicare spending by entire histories of insurance coverage from age 52 through age 64. We could therefore use simple weighted models (marginal structural models) to compare adults who were continuously insured with adults who were continuously or intermittently uninsured before age 65 without bias from observed time-varying confounders.

    We confirmed that fixed and time-varying characteristics in the weighted population before age 65 were similar by insurance status at the time of surveys. To do so, we weighted participants' successive self-reports before age 65 (for example, of general health status or household income) by their inverse-probability-of-treatment weights cumulatively determined through each corresponding survey (the last set of which were used in main comparisons of Medicare spending by previous coverage, as described above) (33). We then conducted pooled comparisons of characteristics from all surveys before age 65 by insurance status assessed in corresponding surveys, and compared differences before and after applying these weights. Relative to unweighted differences, weighted differences between insured and uninsured adults were substantially reduced for all fixed and time-varying sociodemographic and health characteristics. Differences between insured and uninsured adults in general health status, change in general health, mobility, agility, difficulties with activities of daily living, and smoking status were reduced by 66% to 94%. Large differences in household income were almost entirely eliminated, and racial differences were reversed so that uninsured adults were slightly more likely to be white. Each of 14 other categorical sociodemographic and health characteristics that were considered differed by no more than 1 to 2 percentage points in weighted comparisons. Thus, inverse-probability-of-treatment weights evenly balanced fixed and time-varying characteristics between insured and uninsured adults at the time of surveys before age 65 so that differences in health insurance status were not related to preexisting differences in these observed characteristics.

    In the weighted population, uninsurance at any survey wave before 65 was still permitted to predict subsequent health declines that might accumulate and lead to greater Medicare spending after age 65. To characterize some of these potentially mediating effects, we compared health characteristics at the last survey wave before age 65 by longitudinal assessments of coverage (continuously insured adults vs. continuously or intermittently uninsured adults) and weighted these comparisons by participants' last set of inverse-probability-of-treatment weights before age 65 (those used in our main analyses of Medicare spending). Relative to the diminished health differences described above, these weighted differences were 67% to 195% greater, indicating that adults who were uninsured at any survey wave after age 52 subsequently developed worse general health status, more functional limitations, and more difficulties with activities of daily living by their final survey before age 65 than adults who were continuously insured. Thus, these analyses confirmed that our inverse-probability-of-treatment weighting effectively minimized confounding effects of observed time-varying health characteristics on insurance coverage while allowing mediating effects of insurance coverage on health.

    We combined sampling, nonresponse, and treatment weights in the following manner (54–56). First, nonresponse weights were estimated by using a predictive model weighted by survey sampling weights. Second, inverse-probability-of-treatment weights were estimated by using a predictive model weighted by the product of nonresponse and sampling weights. Third, sampling, nonresponse, and treatment weights were multiplied to obtain combined weights used for marginal structural models of the effects of insurance coverage before age 65 on Medicare spending after age 65.

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    Hospitalizations for Exacerbations of Chronic Obstructive Pulmonary Disease

    In contrast to those with cardiovascular disease or diabetes, previously uninsured and insured adults with chronic obstructive pulmonary disease (COPD) may have similar health care needs after age 65 as a result of this condition because treatments for COPD improve symptoms and prevent reversible complications but generally do not alter the rate of disease progression. Specifically, clinical trials suggest that underuse of inhaled therapies by uninsured adults with COPD would increase their risk for exacerbations while they are uninsured but would not accelerate their declining lung function or lead to irreversible complications (57–63). Thus, poor control of this condition before age 65 should not lead to persistently greater health care needs after age 65 if Medicare effectively provides access to appropriate care because regular use of these treatments quickly reduces the risk for exacerbations. We compared adjusted annual rates of hospitalization for COPD exacerbation (DRG 88) among previously uninsured and insured adults who reported a diagnosis of COPD by age 67 or were active smokers at baseline. As hypothesized, previously uninsured adults were not more likely than previously insured adults in this high-risk group to be hospitalized for COPD exacerbations (adjusted odds ratio, 0.52 [CI, 0.27 to 1.01]); P = 0.053). The fact that differences in Medicare spending varied across conditions in a clinically predicted manner strengthens the attribution of higher spending after age 65 to uninsurance before age 65. Furthermore, the non–statistically significant lower rate of COPD exacerbations for previously uninsured adults suggests that they may have had weaker preferences for inpatient services, all else equal, which would tend to bias effects of uninsurance on subsequent Medicare spending toward the null.

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    Estimation of Medicare Cost Offset

    On the basis of the current number of adults age 51 to 64 in the United States (approximately 50.8 million) (64) and the proportions of adults in our study cohort who were continuously or intermittently uninsured (32.0%) and who died (7.6%) before age 65, approximately 15 million adults age 51 to 64 are expected to be uninsured at some point before age 65 and to survive to age 65. For each age from 51 to 64, we calculated the present value of the cumulative predicted difference in Medicare spending from age 65 to 74 between a previously uninsured and insured adult, using a discount rate of 3% and the adjusted annual spending difference estimated in our study ($1023 per person in 2006 dollars). These cumulative differences were adjusted to reflect mortality after age 65 among previously uninsured adults (2.4% annually on average). Differences ranged from $5371 to $7888 because future Medicare spending was discounted more heavily for younger adults than older adults and averaged $6556 per previously uninsured adult. Assuming that the 15 million adults expected to be uninsured before age 65 are evenly distributed across current ages of 51 to 64, the present value of the estimated reduction in subsequent Medicare spending between age 65 and 74 for these adults would total approximately $98 billion if they were continuously insured before age 65.

    On the basis of previously published estimates of differences in adjusted annual per capita medical spending for insured and uninsured adults age 19 to 64 ($1948 per person in 2008 dollars) (16), the proportion of uninsured adults who are age 50 to 64 (17.9%) (16), and per capita spending for near-elderly adults relative to younger adults (approximately 2 times higher) (65), we estimated that providing coverage for uninsured adults over age 50 would increase annual health care spending for these adults by $3304 per person (in 2008 dollars). Assuming that 16.3 million adults age 51 to 64 are or will be uninsured before age 65 (50.8 million × 0.32) for an average of 4 years (based on our study cohort), providing continuous coverage for these adults would increase health care spending before age 65 by an estimated $53.7 billion per year for 4 years. Deflating this annual amount to 2006 dollars and discounting 4 consecutive years of spending to present value yielded an estimated $197 billion in increased spending before age 65 if the current population of adults age 51 to 64 had continuous insurance coverage through age 65.

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    Article and Author Information

    • Acknowledgment: The authors thank Stuart B. Mushlin, MD, for helpful comments on an earlier draft of this manuscript.

    • Grant Support: By a grant from the Commonwealth Fund (20060485).

    • Potential Conflicts of Interest: Consultancies: E. Meara (Employment Policies Institute), J.Z. Ayanian (RTI International, VeriskHealth). Expert testimony: J.Z. Ayanian (U.S. House of Representatives Ways and Means Committee). Grants received: J.Z. Ayanian (National Institute on Aging).

    • Reproducible Research Statement: Study protocol: See Methods and the Appendix, available at www.annals.org. Statistical code: Available from Dr. McWilliams (mcwilliams{at}hcp.med.harvard.edu); SAS code for constructing inverse-probability-of-treatment weights also available in appendix of reference 32. Data set: Health and Retirement Study survey data available at http://hrsonline.isr.umich.edu/index.php?p=data. Linked Medicare claims data are restricted, and use requires approval by the Health and Retirement Study and the Centers for Medicare & Medicaid Services.

    • Requests for Single Reprints: J. Michael McWilliams, MD, PhD, Department of Health Care Policy, Harvard Medical School, 180 Longwood Avenue, Boston, MA 02115; e-mail, mcwilliams{at}hcp.med.harvard.edu.

    • Current Author Addresses: Drs. McWilliams, Meara, Zaslavsky, and Ayanian: Department of Health Care Policy, Harvard Medical School, 180 Longwood Avenue, Boston, MA 02115.

    • Author Contributions: Conception and design: J.M. McWilliams, E. Meara, A.M. Zaslavsky, J.Z. Ayanian.

    • Analysis and interpretation of the data: J.M. McWilliams, E. Meara, A.M. Zaslavsky, J.Z. Ayanian.

    • Drafting of the article: J.M. McWilliams, A.M. Zaslavsky.

    • Critical revision of the article for important intellectual content: J.M. McWilliams, E. Meara, A.M. Zaslavsky, J.Z. Ayanian.

    • Final approval of the article: J.M. McWilliams, E. Meara, A.M. Zaslavsky, J.Z. Ayanian.

    • Statistical expertise: J.M. McWilliams, E. Meara, A.M. Zaslavsky.

    • Obtaining of funding: J.M. McWilliams, J.Z. Ayanian.

    • Administrative, technical, or logistic support: J.M. McWilliams.

    • Collection and assembly of data: J.M. McWilliams.

    Previous Section
     

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